This paper discusses the impact of light coherence on the defocusing
properties of a novel retinal projection display. The display is based
on a liquid crystal display (LCD) illuminated by partially coherent
light from a LED which projects the LCD-image directly onto the eye's retina. It is shown that the increase of the coherence level of the
illumination light enhances on the one hand the contrast of a defocused image. On the other hand, however, the perceived image quality is affected by the occurrence of intermediate defocusing peaks as well as by coherence effects, such as edge-ringing.
Measurements of both the modulation transfer function (MTF) and
of the text readability when defocusing are presented using a bench model of the display system. The experimental results reveal
that for text readability the best DOF can be achieved not for fully coherent illumination, but for partially coherent light of a coherence level σ ≈ 0.35-0.5. Further, it is shown that the optimum σ-level depends slightly on the targeted text size.